i noticed about a week ago that something was going on with my betta. he wasnt as happy and active as normal. then i saw the growth.... im not an expert in fish care, but i have basic knowledge of what to do. im not sure what this is but its freaking me out and it is upsetting sebi. please help!!

It started growing about a week or so ago. Maybe a little longer. No changes in his habitat. I replace half the water every 10 days or so. No changes in his food. He has been in some sort of a mood. Not as active. I don't see. Him ever mess with it. Well at least when I'm watching him he doesn't. I hope it's not cancer I have had him a year on may 4th. Is there anything I could put in his water to help if it isn't a tumor?

probably because their anus is just behind the ventrils and infront of the anal fin and not there where that growth is

id say its cancer... looks like a tumor of some sort...
and if it is there probably isnt anything you can do except keep his water extra clean, add some aq salt, keep him warm and dont change anything like tank or food to avoid stress... nothing else i can think of really...

How's the water-change regimen? Try increasing water changes and adding salt. If that seems like it has an effect on the size of the "growth", continue with that. If not, consider a "one-stop-shop" medication. I don't like to jump right to medications, but that thing looks aggressive.